This parameter as well as the expansion coefficient \(\omega _{10}\)  have been derived from
QCD sum rules [1]}. According to [1]} the uncertainties of these parameters are large
of the order of \(30\%\) . The expansion coefficient \(\omega _{20}\)  has been adjusted by us [3]}
to the CLAS data on \(\pi ^0\)  photoproduction [4]}.
